1. Field of the Invention
The present invention relates to an initiator for polymerizing isocyanates, more particularly, to a metal enolate initiator for polymerizing isocyanates and a method for polymerizing isocyanates by anionic polymerization using the same, in which the initiator forms a cluster upon the initiation and protects stability of terminal anions at the end of the chain to cause controlled polymerization, thus preventing depolymerizaton and improving reaction time and efficiency without the use of a separate additive.
2. Description of Related Art
In general, polyisocyanates are prepared by anionic polymerization [Bur, A. J.; Fetters, L. J. Chem. Rev. 1976, 76, 727.]. Typically, the anionic polymerization is carried out using NaCN as an initiator and N—N-dimethylformamide (DMF) as a solvent at a temperature of −60° C. However, like the polymerization of methylmethacrylate (MMA), which is a polar monomer similar to the polyisocyanate, the anionic polymerization of isocyanates tends to form a thermodynamically stable trimer by a backbiting reaction in which the active anion at the end attacks the carbonyl group of the main chain. As a result, it is difficult to control the molecular weight of a polymer and to obtain a monodispersed polymer. Moreover, the anionic polymerization of isocyanates requires extreme reaction conditions such as high vacuum and low temperature due to a high reactivity of an isocyanate monomer.
A method for controlling the polymerization of isocyanates has been a very difficult problem for numerous researchers, and extensive research aimed at polymerizing isocyanates with a controlled structure by the anionic polymerization has continued to progress.
As an example, Fukuwatari et al. have prepared an anionically polymerized n-hexylisocyanate using lanthanoid isopropoxide as an initiator [Fukuwatari, N.; Sugimoto, H.; Inoue, S. Macromol. Rapid Commun. 1996, 17.]. As a result of the reaction, the polymer was obtained in the yield of 90% at a polymerization temperature of −78° C. While it is very difficult to obtain the polymer at a temperature higher than −40° C. via the general polymerization due to trimerization, a polymer was obtained in a high yield of 73% even at a temperature of −30° C. With this, it was proved that lanthanoid isopropoxide is a more effective initiator than NaCN and also that the polymerization is possible at a relatively higher temperature.
Moreover, Tsuyosi et al. have prepared a homopolymer of hexylisocyanate using yttrium isopropoxide as an initiator at room temperature [Ikeda, T.; Sugimoto, H.; Inoue, S. J. M. S.—Pure Appl. Chem. 1997, A(34), 1907.]. Although this polymerization offered an improved yield, it was difficult to control the molecular weight and to obtain a monodispersed polymer, and thereby it was impossible to find a condition for living polymerization.
Meanwhile, another difficulty raised in the polymerization of isocyanates is solubility to a solvent.
Since the solubility of a monomer and a polymer obtained at low temperature to dimethylformamide (DMF), a commonly used solvent, is not good, the molecular weight distribution becomes broader and the yield decreases. In order to overcome these problems, Okamoto Y. et al. used toluene and DMF as co-solvents. As a result, the yield was improved, but there was no noticeable improvement in the molecular weight distribution. In order to overcome the low solubility problem, the use of tetrahydrofuran (THF) as a solvent was proposed. In this case, however, it was observed that the formation of dimers and trimers became more serious.
Wang J. et al. have synthesized polyisocyanates using SmI2, which is a lanthanide compound [Wang, J.; Nomura, R.; Endo, T. Chem. Lett. 1996, 10, 909: Macromolecules, 1996, 29, 2707.]. However, there were still some drawbacks in that the yield was as low as 32% to 70%, and the molecular weight distribution was as uncontrollable as 2 to 4.
Novak B. M. et al. have prepared a variety of isocyanates by living coordination polymerization using CpTiCL2(OR), etc. [Patten, T. E.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 5065.: Macromolecules 1993, 26, 436.: Macromolecules 1996, 29, 5882.]. In this case, however, there were also some drawbacks in that a catalyst system, which was very complex and expensive, should be used for the coordination polymerization, the yield was still lower than 100%, and copolymerization with other monomers was not possible.
Okamoto Y. et al. have polymerized aromatic polyisocyanates under polymerization conditions of tetrahydrofuran solvent and −98° C., using chiral lithium amides of (S)-(−)-(2-methoxymethyl)pyrrolidone and (S)-(+)-(2-pyrrolidonylmethyl)pyrrolidone, or using chiral alkoxides of 1,2,5,6-diisopropylidene-D-glucose, (−)-menthol, (−)-borneol, and (2S, 3R)-(+)-4-dimethylamino-1,2-diphenyl-3-methyl-2-butanol. However, they did not take into considerations of various parameters for living conditions and for structure-control, such as reaction time, counter ion, and the like.
Meanwhile, the inventors of the present invention have disclosed polyisocyanates having various functional groups and a preparing method thereof in Korean Patent No. 10-0419845, in which a common-ion salt is used to prevent the formation of dimers and trimers due to stable metal ions, thus providing polyisocyanates with controlled molecular weight and improved properties. However, it has a difficulty in the process due to the use of an additive.
Moreover, the inventors of the present invention have disclosed a novel initiator for polymerizing isocyanates having a variety of functional groups in Korean Patent No. 10-0454095, in which a metal amidate is used as the novel initiator to quantitatively obtain a controlled polymer by effectively preventing depolymerizaton without the use of an additive. However, it has some drawbacks such as long reaction time and low initiation efficiency due to a low reactivity of the amidate anion.
As described above, according to the conventional methods for preparing polyisocyanates, it is difficult to control the molecular weight due to the formation of dimers or trimers, and they have some drawbacks such as a difficulty in the process due to the use of an additive, deterioration of reaction yield due to the low reactivity, and a broad molecular weight distribution. Accordingly, a new method for polymerizing isocyanates, which can solve the above-described drawbacks to control the molecular weight and its structure and to and provide improved reaction efficiency, is highly need.
As a result, the inventors of the present invention have confirmed that, if isocyanate monomers having a variety of functional groups including alkyl and aryl are subjected to anionic polymerization using a specific metal enolate compound as a initiator at a high vacuum and an extremely low temperature without the use of an additive, it is possible to effectively prevent depolymerizaton as the initiator molecules form a cluster upon the initiation, to reduce the formation of dimers and trimers, and to facilitate the control of monodispersity and molecular weight, thus polymerizing the polyisocyanate under more stable conditions, and it is also possible to reduce the required reaction time thanks to the reactivity of a carbanion higher than that of the amidate anion of the conventional metal amidate and to improve the initiation efficiency, and completed the present invention.